As a detector for GC, the detectors of various types, such as a flame ionization detector (FID), a pulsed discharge detector (PDD), and a thermal conductivity detector (TCD), have been conventionally in practical use.
Among the aforementioned detectors, the FID is generally used in order to detect organic substances. The FID ionizes sample components in sample gas with hydrogen flames and detects a resultant ion current. The FID has the characteristic of a wide dynamic range, but compounds to be analyzed are limited because the sample is burned in the hydrogen flames to ionize the sample components. The FID has low sensitivity with respect to incombustible gas and no sensitivity with respect to inorganic gas.
In contrast, the PDD, in which the samples are ionized by use of electric discharge, has high sensitivity with respect to the incombustible gas or the inorganic gas, and is suitable for detecting almost all compounds with which the gas chromatograph is required to cope (for example, see Patent Literatures 1 to 4). The method of exciting helium molecules is often used for the generation of plasma, and the PDD in which this method is applied is referred to as a helium discharge photo ionization detector (HDPID).
The HDPID is mainly constituted by a plasma generating section and an ion collecting section.
A plasma excitation electrode is arranged in the plasma generating section. Helium gas is introduced into the plasma generating section, and a high voltage pulse is applied to the plasma excitation electrode, so that the helium gas is excited, thereby generating the plasma. The light (vacuum ultraviolet light and the like) emitted by the plasma reaches the ion collecting section.
An ion collecting electrode and a bias electrode are arranged in the ion collecting section. The sample gas is introduced into the ion collecting section, and the light reached from the plasma generating section is cast to the sample gas, thereby ionizing the sample gas (sample ions). Voltage is applied to the bias electrode, generating an electric field, and the sample ions are guided to the ion collecting electrode. The ion collecting electrode collects the sample ions and detects the sample ions as an ion current through an amplifier connected to the ion collecting electrode.
An insulating member of, for example, ceramic (having thickness of several millimeters) is inserted between the ion collecting electrode and the bias electrode to electrically insulate one from the other (for example, see Patent Literatures 1 to 4).